1. Field of the Invention
This invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns an improved liquified natural gas (LNG) facility employing one or more liquid expanders for reducing the pressure of a process stream and generating electricity that is used to at least partially power a compressor located elsewhere in the facility. In still another aspect, the invention relates to a method and apparatus for reducing the pressure of a refrigerant stream in one of the closed or open refrigeration cycles in the LNG facility using a liquid expander, and generating electricity through this expansion to at least partially power a compressor situated in a location within the LNG facility that is remote from the liquid expander.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
Natural gas is frequently transported by pipeline from the supply source to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
As is typical with numerous processes of this type, the cooling of high-pressure streams can be achieved through the flashing or rapid expansion of the stream. This expansion is commonly affected through the use of joule-Thompson (J-T) expansion valves. The use of J-T valves results in the adiabatic expansion of the stream. Other types of equipment, such as liquid expanders, can be used to perform this expansion. Liquid expanders generally result in tropic expansion and have the benefit of producing work as the stream passes therethrough. This work can be harnessed via a shaft connected to another piece of equipment such as a compressor. The main disadvantage with this type of direct mechanical coupling of an expander and a compressor is that both pieces of equipment must be located in very close proximity to one another. Therefore, such optimizations must be considered when originally designing the LNG facility in order to most efficiently situate the equipment and conduit lines. It is difficult to retrofit an existing facility with this type of direct mechanically coupled expander/compressor arrangement due to the fact that the high-pressure stream may not be located near the stream needing to be compressed. Therefore, a real need exists for a method and apparatus for enabling the extraction of energy from high-pressure streams in an LNG facility and using that energy elsewhere in the facility without necessitating major reconfigurations of the plant design so that the expander and compressor can be positioned in close proximity to each other.